Infrared thermometers
measure the temperature of an object without
touching it. It is therefore possible to perform fast, reliable
temperature measurements of moving, hot or difficult-to-access
objects. While contact temperature sensors or probes can influence
the temperature of the target object, sometimes even damage the
product itself, the noncontact method ensures precision measurements
without damaging the target object. Infrared sensors can also
measure very high temperatures, whereas a contact sensor would
either be destroyed or would have a shorter service life.

Not only are infrared devices now relatively inexpensive, they
also offer numerous technical benefits and a variety of options
for users, including handheld or inline process control, open
connectivity to fieldbus systems and options for hazardous environments.

For accurate temperature measurement using infrared sensors,
users must carefully consider two key parametersemissivity and
wavelength.

Emissivity
All bodies above absolute Kelvin (-273°C) emit infrared radiation
in three ways, via a combination of emitted radiation, radiation
reflected from the surroundings, and by transmitting the radiation
through itself. How these factors interact depends on the material
of the measurement object. However, for non-contact infrared
temperature measurements, only the emitted radiation element
is important.

The relationship of the emission types to each other is best
described in the following way. If at any given temperature,
the sum of the radiation of the three emission types is equal
to one, and it is assumed that solid bodies transmit negligible
radiation, the transmitted element can be treated as zero. Therefore,
the heat energy coming from an object only comprises emitted
and reflected radiation.

This is why objects such as polished and shiny metals can only
have a low emission, or emissivity, as radiation from the surrounding
environment is strongly reflected (and so proportionally high)
by these surfaces.

Wavelength
The emissivity of an object will vary when monitoring the radiated
heat energy at different wavelengths. Therefore, developing sensors
that measure temperature at specific wavelengths can significantly
increase measurement stability.

Material groups can be used to describe the optimum wavelengths
for highest object emissivities and therefore the most stable
results. For metals, 0.8 to 2.3µm, glass 5µm, textiles
and most matt surfaces 8-14µm. Thin film plastics are more
complex, requiring specific wavelength sensors to be developed
for polyethylene, polypropylene, Nylon and polystyrene (3.43µm).
Polyester, polyurethane, Teflon, FEP and polyamide require 7.9µm.
Thicker, pigmented films require 8-14µm.

When selecting an infrared temperature sensor, it is vital that
the wavelength band over which the sensor measures is known,
and that the correct wavelength band is used for the object to
be measured.The object emissivity values over this wavelength
and the temperature range to be measured must also be known or
calculated.